All I was able to learn, after several weeks, is that it’s a very common problem (which is an understatement). At this time, here’s what we seem to have learned:

Band 2 Cortana works fine with iOS, Android, and Windows Phone 8.1 devices.

Band 2 Cortana doesn’t work with Windows 10 Mobile devices.

I took my fully updated Lumia 950 XL and Band 2 to my local MS store after scheduling an Answer Desk appointment. I was told that this has been a known issue since November and that there seems to be very little discussion on it since then… it seems to have stalled.

Also, since my initial post, Band 2 and Microsoft Health apps have been updated and installing these updates have not resolved the issue.

It’s troubling to me that such core functionality of flagship products has gone unrepaired for six months!